General methods of completion of production wells in heavy oil or tar sand reservoirs, as practiced until a few years ago, are generally not pertinent to more recent work, including the invention disclosed below. A reference which appears to be of other than general significance is the L. E. Elkins U.S. Pat. No. 3,504,745. This patent teaches minimizing vertical passage of fluids outside a well by injecting into the path (which would otherwise be followed by such fluids) a foaming agent which can, for example, be an aqueous solution of any of a number of cited soaps, at a concentration in the order of 1 to 2%.
The T. S. Buxton, et al. U.S. Pat. No. 3,399,722 teaches creating separate upper and lower sets of perforations into a reservoir in a tar sand or heavy oil region. First, a zone of high permeability is created by combustion through the perforations at the lower part of the zone. After this has been carried on for several days, the zone is killed and the upper zone created by perforating. Production of heated material from the reservoir occurs through the upper zone. Accordingly, only one of these two zones (upper and lower zones of the reservoir) is used at one time. Our process intentionally uses flow of quite dissimilar fluids for different purposes into each of the two zones (upper and lower) simultaneously, in order to condition the production well for use in our invention.
R. M. Jorda shows a production well assembly for in situ combustion operations in U.S. Pat. No. 3,160,208. A number of perforations extend through the walls of two casing strings into a formation to be produced. Production resulting from in situ combustion enters these conduits and can be pumped from the well. Hot produced gases can flow out of the well through the annulus between a production string and the inner casing string. However, the inventor does not discuss means of conditioning the well prior to its use for ordinary production.
B. G. Harnsberger in U.S. Pat. No. 4,066,127, teaches circulating hot fluids out into the formation through a set of upper perforations into a reservoir and back through a set of lower perforations to form a void in the tar sands. This is followed by gravel packing the void, and injecting further hot fluids through the upper perforations to flow heated organic material from the reservoir through the gravel pack and a sand screen. This involves several disadvantageous procedures compared with ours. We provide for only outflow through the lower perforations, and never create a void in the reservoir by a melting process. This creates too many problems of sand movement through and near the void--and sand control is vital in production of tar sands and heavy oil from the usual unconsolidated reservoirs. There are other differences, but this is sufficient to show that these are quite different processes.
Finally, R. B. Needham in U.S. Pat. No. 4,068,717, provides a method for tar sands reservoir production using the difficult practice of employing steam to fracture from an injection to a production well in the reservoir. These steps do not otherwise condition the production well (which is the object of our invention). He uses the injection of steam, accompanied by a surface-active agent, to produce the reservoir, rather than a frontal thermal drive as employed by us.
It is thus apparent that these literature references considered alone or together, do not teach or suggest the essence of our invention, as summarized below.